At 08:52 AM 3/17/2007, Robert G. Brown wrote:
>As I'm working up a server room and some UPS's, a question arises. The
>APC units we have have a different plug and current rating from the
>circuits in the wall. In particular, we have L6-30R receptacles and the
>APC requires only 20A. I had thought that it would be perfectly safe to
>replace an L5-20P with an L6-30P and plug a 2200 VA UPS into a 30 amp
>circuit, but the electrician who was doing some of the rewiring said "we
>can't do that" because "the 30A circuit breaker might not blow in time
>to protect" the 2200 VA device from damage in the event of an internal
>short.
First off, the NEC really only deals with the permanent side of
things.. (the utilization equipment side is dealt with by someone
like UL or some other NRTL, and they would impose requirements for
size of wire, etc.)
here's the deal on "receptacle ratings" from the NEC, bearing in mind
that your local AHJ (Authority having Jurisdiction) may be different,
and ultimately it's up to the inspector:
210.21 (B)(1) Single receptacle on an Individual branch Circuit
A single receptacle installed on an individual branch circuit shall
have an ampere rating not less than that of the branch circuit.
Exception 1: In accordance with 430.81(B)
Exception 2: A receptacle installed exclusively for the use of a
cord-and-plug connected arc welder shall....
(2) Total Cord and Plug connected load
<snipped>
(3) Receptacle Ratings. Where connected to a branch circuit supplying
two or more receptacles or outlets, receptacle ratings shall conform
to the values listed in Table 210.21(B)(3), or where larger than 50
amperes, the receptacle shall not be less than the branch circuit rating.
Table 210.21(B)(3)
Circuit Rating Receptacle Rating
15 not over 15
20 15 or 20
30 30
40 40 or 50
50 50
(430.81 has to do with motor controllers less than 3/4 hp)
>This argument seems quite senseless to me. If this argument were
>correct we could never plug a 15 A device into a 20 A circuit, etc. It
>would really simplify my life if I can just rehead the cable. Do any of
>the electricity-savvy people here have any comments regarding the safety
>or legality of doing so? I have always assumed that going up a size in
>circuit capacity is generally safe and legal...
Did you increase the size of the conductors in the cord to match the
new plug? Is the fault interrupting capability inside the equipment
rated to handle the higher peak fault current available from a higher
rated branch circuit?
This is known as "coordination" of overcurrent protection (i.e. you
can't put a big 100 A plug on 18 gauge lamp cord)
The NEC does have a bit to say about flexible cords and
cables. 400.5 talks about minimum conductor sizes. If you have only
2 current carrying conductors in your cable, you can get away with
AWG10 wiring in the cable. If you have 3 current carrying conductors,
you'd need AWG8.
Now, you might be subject to the rules in Art.645 Information
technology Equipment.
For instance, 645.5 requires that the ampacity of the branch circuit
conductors be rated at not less than 125 percent of the total
connected load (i.e. 2200 VA * 1.25, i.e. 25 A or so)
Also, you need to be aware of the requirement for a single switch
that can disconnect power to all electronic equipment in the room
(645.10) This also applies to the loads from the UPSes (actually, I
think that you also need to have a "disconnecting means" to
disconnect the battery from the load) (i.e. the "big red switch" at
the door). UPSes rated at less than 750 VA (I think) are exempt from
this requirement, but your 2.2kVA units will probably be subject to
it. You might be able to have multiple controls, but they need to be
grouped, and by the door.
{this is so that the fire fighters can hit a button and KNOW that
everything in the room isn't still live}
>I'm planning to do the rewiring of cables today soon, so if you know
>something about his please respond "soon".
>> rgb
>>--
>Robert G. Brown http://www.phy.duke.edu/~rgb/>Duke University Dept. of Physics, Box 90305
>Durham, N.C. 27708-0305
>Phone: 1-919-660-2567 Fax: 919-660-2525 email:rgb at phy.duke.edu>>>_______________________________________________
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